In 3GPP R11, the design of an enhanced physical downlink control channel (ePDCCH) is based on considerations from Enhanced Inter Cell Interference Coordination (eICIC), and enhancement new carrier types such as CoMP and DL MIMO in the current protocol. In 3GPP RAN#70 Meeting, the ePDCCH set has been publicly known as a key feature in ePDCCH design, for example:
I. An ePDCCH set is defined as a group of N PRB pairs:                A distributed ePDCCH is transmitted using the N PRB pairs in an ePDCCH set;        A localized ePDCCH should be transmitted within an ePDCCH set;        
II. A base station configures K≧1 ePDCCH sets for the UE:                Maximum number for K is selected among 2, 3, 4 and 6;        The total number of blind decoding attempts is independent from K;        The total blind decoding attempts for a UE should be split into configured K ePDCCH sets;        Each ePDCCH set is configured for either localized ePDCCH or distributed ePDCCH;        PRB pairs of ePDCCH sets with different logical ePDCCH set indices can be fully overlapped, partially overlapped, or non-overlapping.        
According to the above currently agreed definition of an ePDCCH set, a UE (User Equipment) configured with ePDCCH will monitor a group of PRB pairs with several ePDCCH sets for USS (UE-Specific Search Space) detection. Part of the ePDCCH sets are for localized transmissions, while the remainings are for distributed transmissions. The ePDCCH sets for distributed or localized transmissions may be partially overlapped, fully overlapped, or non-overlapped.
The current protocol provides a flexible configuration of PRBs for UE to monitor ePDCCH USS. According to this protocol, the legacy hash function (depiction of the hash function is provided in section 9.1.1 in 3GPP TS 36.213) method, for example equation (1) and equation (2), for ePDCCH searching space cannot be used here without any modification.L{(Yk+m)mod └NCCE,k/L┘}+i  (1)Yk=(A·Yk-1)mod D  (2)Wherein L denotes an aggregation level, m is the candidate number for each aggregation level L, NCCE,k is the total CCE number in the control region of the subframe K, Yk is defined as i=0, . . . , L−1, m=0, . . . , M(L)−1, and M(L) denotes a candidate number of PDCCHs which need to be monitored for a given searching space.